The present invention relates to monitoring subterranean formations and more particularly, systems and methods for enhancing reservoir characterizations using real-time parameters.
Monitoring of reservoir behavior due to injection and production processes is an important element in optimizing the performance and economics of completion and production operations. Examples of these processes may include hydraulic fracturing, water flooding, steam flooding, miscible flooding, wellbore workover operations, remedial treatments and many other hydrocarbon production activities, as well as drill cutting injection, CO2 sequestration, produced water disposal, and various activities associated with hazardous waste injection. Because the changes in the reservoir may be difficult to resolve with surface monitoring technology, it may be desirable to emplace sensor instruments downhole at or near the reservoir depth in either special monitor wells or within the injection and production wells.
The following description relates to identifying a stimulated reservoir volume (SRV) for a stimulation treatment of a subterranean region. Microseismic data are often acquired in association with injection treatments applied to a subterranean formation. The injection treatments are typically applied to induce fractures in the subterranean formation, and to thereby enhance hydrocarbon productivity of the subterranean formation. The pressures generated by the stimulation treatment can induce low-amplitude or low-energy seismic events in the subterranean formation, and the events can be detected by sensors and collected for analysis. The purpose of the hydraulic fracturing is to induce an artificial fracture into the subsurface, by injecting high pressured fluids and proppants into the rock matrix, in order to enhance the productivity of the reservoir for hydrocarbons.
The microseismic event locations are commonly monitored in real-time and the locations of events shown in a three-dimensional (3D) view may be validated as they occur. They are also available for analysis after the conclusion of the hydraulic fracturing treatment and are thus, available to be compared to the results of other wells in the area. The microseismic events usually occur along or near subsurface fractures that may be either induced or preexisting natural fractures that have been reopened by the hydraulic fracturing treatment. The orientation of the fractures is strongly influenced by the present-day stress regime and also by the presence of fracture systems that were generated at various times in the past when the stress orientation was different from that at the present.
Each separate and distinct microseismic event that is detected and analyzed is the result of a downhole fracture, which has an orientation, magnitude, location, and other attributes that can be extracted from a tiltmeter or seismic sensor data. The fracture may be characterized with other parameters such as length, width, height, and pressure, for example. There is a location uncertainty associated with each microseismic event. This uncertainty is different in the x-y direction than it is in the vertical (z) depth domain. The location uncertainty of each event may be represented by a prolate spheroid.
In some cases, there is an obvious orientation and spacing of microseismic events that follows the classical bi-wing fracture concepts that are often used in mathematical depictions of fracture analysis. In other cases, a dense data cloud, which represents the 3D volume that encompasses all of the microseismic datapoints, is evidence of a complex fracture pattern of induced or reactivated fractures. In these cases, the analysis of the microseismic data becomes very subjective and interpretive. Even in these cases, there are patterns within the data cloud that may be representative of the fracture patterns that are present in the subsurface.
The stress field today may be different from the one at the time of the original fracture creation. The present-day orientation of the induced hydraulic fractures is strongly influenced by the stress rate in the subsurface. There is always some degree of stress anisotropy between the vertical stress and the two horizontal stresses. The greater the anisotropy, the more planar the fractures that are induced by hydraulic fracturing stimulation and the more they will fit the traditional bi-wing model. The greater the permeability of the rock, the more planar the fractures will be. The more isotropic the stress regime, the more the fractures can be easily deflected by discontinuities in the rock and can create a complex fracture network.
Currently, there are several fracture characterization techniques that have been used to try and identify the orientation, dip and spacing of induced and natural fractures.
In one technique, the overall data cloud of microseismic datapoints is identified to build a stimulated reservoir volume, SRV or estimated stimulated volume, ESV. The information is inferred to be a measure of the amount of rock that has been stimulated by the fluids and proppants. Only a small portion of the energy that is pumped into the ground, however, is ever received at the surface as detectable microseismic events.
There are also several different fracture characterization techniques that are able to mathematically associate the microseismic event data with a model of the subsurface and produce a discrete fracture network (DFN) or a set of probably fracture characterizations such as, for example, the techniques described in U.S. Patent Application Publication Nos. 2010/0307755 and 2011/0029291.
These and other techniques however, suffer problems in that the data analysis could be implicated if a microseismic event based on fluid being pumped due to first fracture parameter appears in subsequent fracture parameters. Additionally, the stimulated reservoir volume can be further enhanced by details that correlate the microseismic event data to its real-time analysis.
While embodiments of this disclosure have been depicted and described and are defined by reference to exemplary embodiments of the disclosure, such references do not imply a limitation on the disclosure, and no such limitation is to be inferred. The subject matter disclosed is capable of considerable modification, alteration, and equivalents in form and function, as will occur to those skilled in the pertinent art and having the benefit of this disclosure. The depicted and described embodiments of this disclosure are examples only, and not exhaustive of the scope of the disclosure.